示例：（内）角平分线定理

The angle bisector theorem is a formula having to do with a triangle split in half by a line bisecting (halving) one of its interior angles and cutting the opposite side. This formula is the relationship between the line segments created by the angle bisector line and the other two sides.

Above is a random triangle above with an angle bisector $CD$ splitting $\angle ACB$ in half. This procedure creates two triangles, $\bigtriangleup ACD$ and $\bigtriangleup DCB$. By the Angle Bisector Theorem, the ratio of side $CA$ to segment $AD$ is the same ratio as side $CB$ to segment $DB$.

Good luck with the Angle Bisector Theorem!